Measuring OPAMP input bias current

[movinghoon96]

Hello, I'm trying to measure Input bias current of TI LM741.
I understood how to measure Vos, but being confused on next step

does 'Input bias current' meaning the current flowing out from V+, V- terminal?
LM741 has NPN BJT for Input Terminal, so I thought it was opposite direction.

According to

Simple Op Amp Measurements | Analog Devices

Op amps are often used in high precision analog circuits, so it is important to measure their performance accurately.

www.analog.com

https://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf

It seems like current through R7 should be supplied from V- terminal

 

[KlausST]

HI,

the description is rather clear.

You have two bias currents:
* on IN- (= IB-)
* and on IN+ (= IB+)

The difference between both should be the offset current.

Klaus

 

[danadakk]

does 'Input bias current' meaning the current flowing out from V+, V- terminal?

The inputs are NPN and Ibias, not to be confused with Ioffset, is into the base.

It seems like current through R7 should be supplied from V- terminal

The way test setup is done is inputs biased at ~ 1/2 the CMR (Common Mode Range),
eg the operation range of the inputs. Note the actual CMR of the 741, is not exactly
symmetrical, but when OpAmp power from split supplies using ~1/2 way or ground
perfectly acceptable for measuring Ibias. And if you look at datasheets at 15V supplies
the vendors, largely, fudged the CMR as symmetrical, even though in realty there is a
difference due to input structure and tail currents/circuitry.


Regards, Dana.